Lubricating oils for internal combustion engines of automobiles or trucks are subjected to a demanding environment during use. This environment results in the oil suffering oxidation which is catalyzed by the presence of impurities in the oil such as iron compounds and is also promoted by the elevated temperatures of the oil during use. This oxidation of lubricating oils during use is typically controlled to some extent by the use of antioxidant additives which may extend the useful life of the oil, particularly by reducing or preventing unacceptable viscosity increases.
We have now discovered that a combination of about 50 to 1000, preferably 50 to 500, more preferably 50 to 250, parts per million (ppm) of molybdenum, based on the total weight of the finished lubricating oil composition, from an oil-soluble molybdenum compound which is substantially free of reactive sulfur; from 1,000 to 20,000, preferably 1,000 to 10,000, ppm of an oil-soluble diarylamine; and from 2,000 to 40,000 ppm of an alkaline-earth metal phenate, is highly effective in inhibiting oxidation in lubricant compositions and providing the lubricating oil with excellent sliding friction characteristics that reduces tappet wear and valve and piston deposits in gasoline, diesel and natural gas (NG) engines.
Lubricant compositions containing various molybdenum compounds and antioxidants, such as aromatic amines, have been used in lubricating oils for some time. Such prior compositions include active sulfur or phosphorus as part of the molybdenum compound, use additional metallic additives or various amine additives which are different from those used in this invention, and/or have concentrations of components that are different than those disclosed by this invention.
Engines have been designed and built specifically for natural gas (NG). These engines are used primarily in stationary applications and are operated under relatively constant operating conditions. Most recently there have been applications of compressed natural gas (CNG) in motor vehicles, especially buses and fleet trucks, due to the economic and environmental benefits associated with NG.
While the basic designs for stationary NG engines and conventional fueled engines (diesel and gasoline) are similar, the differences in operating conditions and maintenance practices have resulted in two distinct lubricant product groups. Stationary NG engine lubricants are usually high viscosity monograde formulations with a low ash content. Conventional fueled engines for vehicles typically use multigrade oils with much higher ash content. The needs of NG engines in transportation applications have not been adequately met by the lubricants presently available and a need exists to design lubricant products that simultaneously fulfill the performance criteria of NG engines in non-stationary applications, gasoline engines and diesel engines. Gasoline and diesel vehicular lubricants are often qualified based on dynamometer tests in a relatively short period of time based upon substantial field experience. However, with the use of an alternative fuel, such as NG, the possibility exists that the performance of accepted oil additives for conventionally fueled engines will be very different in the NG setting. None of the prior art lubricant compositions are directed to solving the special lubricant problems associated with NG engines.